1. Field of the Invention
The present invention is in the field of hydraulics and more specifically relates to hydraulic positioning devices of a locking type.
2. The Prior Art
The use of hydraulic locking devices for positioning adjustable apparatus is widespread. For example, hydraulic locking devices are used on aircraft seats to permit the tilt of the seat back to be adjusted at will. The hydraulic positioning device basically includes a closed cylinder filled with hydraulic fluid, within which a piston is selectively movable. Movement of the piston within the cylinder is enabled by manual actuation of a check valve which, in its normally closed position blocks the flow of hydraulic fluid from one side of the piston to the other side of the piston.
Although the hydraulic positioning device can be connected to the seat in a number of ways, it is usually connected so that as the seat back is tilted backward, the device is compressed, with the piston and piston rod being forced into the cylinder.
For logistic reasons it is desirable to use a single model of hydraulic locking device on all of the seats of an airplane. However, it may be desirable to limit the extent of the tilt of some of the seat backs to prevent them from locking emergency doors or hitting against bulkheads. Thus, there is a need for an adjustable stop which can be used selectively to set the maximum angle of tilt of the seat back. It is desirable that the stop be readily adjustable by authorized personnel, but otherwise tamperproof.
In a typical application, as the seat back is tilted backward, the hydraulic locking device is compressed. As typically mounted, the piston rod remains stationary and the cylinder advances over it.
It is known in the prior art to provide a fixed stop connected to the piston rod to limit the motion of the cylinder as it advances over the stationary piston rod. While such a stop is beneficial in preventing damage to the piston and the cylinder and in limiting the motion, it suffers from the disadvantage of not being readily adjustable.
In U.S. Pat. No. 3,177,980 issued Apr. 13, 1965 to P. L. Porter, there is disclosed a selectively adjustable stop for a hydraulic locking device. The stop includes a pin perpendicular to the axis of the device and affixed to the piston rod. The outer portions of the pin move axially in slots in an extended portion of the cylinder wall. Several slots of different lengths are provided and these are selectable by rotating the extension of the cylinder wall about the axis.
A stop providing a continuous range of adjustment is also known in the art. It includes a cup-like cap, affixed to the stationary piston rod with the open end of the cup disposed to receive the advancing cylinder. The cup-like cap is threaded on its inside cylindrical surface. A threaded stop member engages the threads of the cap and engages an axially-extending groove in the piston rod so that as the cap is rotated on the piston rod, the stop member, which does not rotate advances along the piston rod providing in effect an adjustable bottom in the cup-like cap for stopping the advancing cylinder. Although this type of stop is continuously adjustable, it is also easily tampered with. Further, it has proven expensive to produce since high-load-bearing acme threads are used inside the cap and because of the difficulty of machining the axially-extending groove in the normally threaded piston rod.
For the uses contemplated, such as the adjustment of seat back tilt, a continuous range of adjustment is not required. As will be seen below, the present invention provides a relatively less expensive stop selectively adjustable to any of several discrete positions.